1. Field of the Invention
The invention relates to the field of printing systems and in particular relates to printing systems having dual/tandem print engines using LED array exposure technologies that correct for shrinkage of the printable medium from the first print engine.
2. Statement of the Problem
In high performance printing systems, which can be continuous form printing systems or cut sheet printing systems, the image marking engines apply RIPped (e.g., rasterized) images to continuous form paper moving through the marking engine at high rates of speed. Most such printing system use electrophotographic techniques to apply a rasterized image to the printable medium as dots or pixels of toner particles transferred to the medium and fused thereto typically by heat and pressure. A photoconductive surface is controllably exposed to create a latent image thereon represented as charged and discharged points or pixels. Charged toner particles are then transferred to the photoconductor to develop the latent image and the developed image is then transferred to the printable medium.
In one common configuration, dual or tandem print engines receive the continuous form printable medium whereby a first print engine applies images to a first side of the continuous form printable medium. A second print engine receives the output from the first and applies a second image to a second side. Often, such tandem print engines are physically configured such that the continuous form medium printable medium flips over after it exits the first print engine and is about to enter the second print engine.
In another common configuration, each of multiple print engines may print a different image on the same side of the printable medium. For example, a first print engine may imprint a first color image on a side of the printable medium and the second print engine may imprint a second image of a different color on the same side of the printable medium. Often the first color may be used for text while the second color may be used as a highlight color.
In still other configurations, full four color printing may be achieved by imaging each of the four standard colors in a sequence of four print engines. The continuous form printable medium may pass from the first to the second, to the third, and to the fourth print engines each imprinting a different color and each print engine fusing or fixing its respective image to the same image side of the printable medium.
It is generally known in such dual/tandem print engine configurations that the printable medium (typically paper) may shrink especially in its width dimension (orthogonal to the process direction of paper movement) upon exit from the first print engine. Typically such shrinkage is caused by the heat and pressure used by the print engine to fix or fuse the transferred, toned image onto the printable medium. Such heat and pressure may cause the printable medium (particularly paper) to shrink in the width dimension in which it is not constrained.
Such shrinkage of the printable medium gives rise to problems of alignment and scaling accuracy of the first images applied to the printable medium versus subsequent images applied to the printable medium—whether on the same side or on opposite sides of the printable medium. Frequently printing applications require a high degree of correlation between the size and position of images imprinted on both sides of the continuous form printable medium. Or, a high degree of precision in aligning/registering is required for multiple images all printed in sequence on the same side of the printable medium.
When the print engines use laser printhead mechanisms to expose the latent image on the photoconductor, the laser optics mechanism may be easily altered with respect to its timing on the second print engine to compress the exposed image and thereby compensate for shrinkage of the printable medium upon exit from the first print engine. Other prior solutions include applying multiple developed images to either or both sides of the continuous form printable medium and then fusing or fixing all the distinct images simultaneously so that differential shrinkage is not a factor in differentiating the front side from a backside or a first image from a subsequent image applied to the same side. However, such fusing techniques are complex and the un-fused images require careful, complex handling to avoid destruction of the image.
Still other prior techniques require pre-treating the printable medium before entering the first print engine so that no further shrinkage will occur upon exit from the first printing engine. Rather, all expected shrinkage is provided by the pre-treatment (e.g., heating) process prior to printing either side of the continuous form printable medium.
Presently practiced solutions for such a problem limit the use of LED array printheads in which the pixel spacing is fixed by the static LED spacing of the array. Without resorting to expensive, complex solutions such as pre-treatment or simultaneous fusing of both sides, LED array dual print engine system suffer from differential shrinkage on the two sides of the printable medium.
It is evident from the above discussion that a need exists for an improved method and associated systems for adapting the printing system to adjust for shrinkage of the printable medium upon exit from the first print engine of a dual/tandem LED-based printing system.